At a conference I recently attended, I heard a rumor that the ABA was going to issue an opinion saying it was okay to view metadata sent in a document outside the context of discovery between lawyers. Further, the rumor is they'll say that it's unethical not to scrub it out before you send it.
I've spoken to hundreds, if not a thousand or more, lawyers from small and medium size firms around the country about this, and the ABA's position is, imho, premature at best. Until there's a larger body of lawyers who know about metadata, it should be presumed that its transmission was not intentional; putting the rule the way the ABA apparently intends to do it will result in sharp tactics by large firm lawyers, and little else. Bad idea, ABA, bad idea! (Hopefully the rumor was wrong!)